The field of rapid prototyping involves the production of prototype articles and small quantities of functional parts, as well as structural ceramics and ceramic shell molds for metal casting, directly from computer-generated design data.
Two well-known methods for rapid prototyping include a selective laser sintering process and a liquid binder three-dimensional printing process. These techniques are similar, to the extent that they both use layering techniques to build three-dimensional articles. Both methods form successive thin cross-sections of the desired article. The individual cross-sections are formed by bonding together adjacent grains of a granular, i.e., particulate, material on a generally planar surface of a bed of the granular material. Each layer is bonded to a previously formed layer to form the desired three-dimensional article at the same time as the grains of each layer are bonded together. The laser-sintering and liquid binder techniques are advantageous, because they create parts directly from computer-generated design data and can produce parts having complex geometries. Moreover, three-dimensional printing can be quicker and less expensive than machining of prototype parts or production of cast or molded parts by conventional “hard” or “soft” tooling techniques that can take from a few weeks to several months, depending on the complexity of the item.
An early three-dimensional printing technique, described in U.S. Pat. No. 5,204,055, incorporated herein by reference in its entirety, describes the use of an ink-jet style printing head to deliver a liquid or colloidal binder material to sequentially applied layers of powdered material. The three-dimensional ink-jet printing technique or liquid binder method involves applying a layer of a powdered material to a surface using a counter-roller. After the powdered material is applied to the surface, the ink-jet print head delivers a liquid binder in a predetermined pattern to the layer of powder. The binder infiltrates into gaps in the powder material and hardens to bond the powder material into a solidified layer. The hardened binder also bonds each layer to the previous layer. After the first cross-sectional portion is formed, the previous steps are repeated, building successive cross-sectional portions until the final article is formed. Optionally, an adhesive can be suspended in a carrier that evaporates, leaving the hardened adhesive behind. The powdered material may be ceramic, metal, plastic or a composite material, and may also include fibers. The liquid binder material may be organic or inorganic. Typical organic binder materials used are polymeric resins or ceramic precursors, such as polycarbosilazane. Inorganic binders are used where the binder is incorporated into the final articles; silica is typically used in such an application.
Existing 3D printing materials offer fast, low cost methods for producing prototypes and concept models. Three-dimensional printing enables the formation of full color three-dimensional parts in one production operation. Many existing materials, however, have drawbacks, such as low handling strength, the need for the user to infiltrate the green part (i.e., as made by the machine) to increase strength, and the quality of the color created.
Color accuracy during three-dimensional printing affects the utility of a product. As color quality approaches that of the computer screen, designers, architects, etc., can create computer files, put the labels and shading directly on the model, and skip the steps of printing and applying labels, having the models painted, etc. One aspect of color accuracy, similar to paper printing, is the requirement to have a bright, white, neutral substrate. In three-dimensional printing (also referred to herein as “3D printing”), the models have much higher porosity than paper printing, and the substrates are not prepared in a layered fashion prior, then dried, prior to applying the inks. It is necessary to choose appropriate starting materials, i.e., powder, ink, and infiltrant, so that they combine to form a white part or article in their reacted state in order to create a high quality color part. It is not sufficient to merely include a high dose of any white pigment to achieve the brighter white. Loading a powder formula with white pigment does increase the whiteness of a printed part, but at the cost of a loss of the darker, more saturated color in the gamut.
Another feature of existing three-dimensional printed articles, especially those made out of plaster-based systems such as Z Corporation's zp130, is that the performance of the final, infiltrated strength of the article may vary with ambient conditions and the viscosity of the infiltrant. One common infiltrant is Z Corporation's zbond101, a cyanoacrylate-based adhesive. Similar products are made by many companies, such as Loctite. Penetration of the infiltrant can be reduced in humid conditions where printed parts do not fully dry, or when the infiltrant has aged (therefore increased in viscosity). Reduced penetration leads to an effectively weaker prototype. The variability in performance may also be a source of frustration, as a user may be able to use his prototypes for his application during one art of the year, but not during another.